The present invention relates to means for protecting bearings, shafts, rolls, and other cylindrical surfaces against wear and/or corrosion. Numerous means are known for protecting cylindrical surfaces. One common method is to construct the cylindrical component from wear/corrosion resistant materials. Another common method is to apply protection to specified areas of the component via welding, spraying, spraying/fusing, brazing, heat treating, etc. Usually, cost prohibits the widespread use of the first option. The second option involves problems with dimensional distortion, as application of the protective layer involves exposure of the component to elevated temperatures, and the component and protective layer have differences in thermal expansion. If the coating process does not include some form of fusing/bonding operation, the resulting product suffers from poor bond strength and is prone to spalling during service.
Another method of wear/corrosion protection is the attachment of protective cladding onto a component via interference fitting, adhesives, fasteners, etc. However, in the case of wear, most materials which provide adequate protection, such as sintered carbides and ceramics, are hard (over 40 Rc) and brittle. Due to the lack of mechanical strength, their ability to transfer mechanical stresses to the coupling component without defect initiation and catastrophic failure is very limited. Also, the ability to put these brittle clads onto outside diameters, which creates tensile stresses by interference fitting, is limited.
In the exploration for oil and other mining operations, mud motors are used extensively. The mud motor consists of three sections: the power section, the bearing assembly, and the bit box. Within the bearing assembly, there are several radial bearing sets and multiple thrust bearing sets on a drive shaft. Typical motors have two pairs of radial bearings consisting of the upper rotating and stationary and lower rotating and stationary pair. Due to the harsh conditions to which these bearings are subjected, they require frequent replacement and are a major portion of the motor's operating cost.
During the drilling operation, the bearings are lubricated with a specialized drilling mud. Rock cuttings from the operation frequently contaminate the system, thus destroying the lubrication properties of the mud. As a result, the bearings suffer wear through abrasion and erosion. Additionally, bearing-to-bearing contact due to restricted mud flow and normal service requirements create severe galling wear. In addition to wear, high mechanical stresses in the form of cyclical loading, tensile overload and high impact cause fatigue cracks and, eventually, catastrophic failure. Well bores also tend to be hot (in excess of 400.degree. F.). Under these conditions, the lubricating mud may become corrosive due to reservoir chemical contamination. All of these factors combine so that a successful bearing must be wear resistant with poor lubrication, mechanically tough, impact resistant, corrosion resistant, and able to operate at elevated temperature.
A number of different types of materials in the form of bearings and bearing sleeves have been used to increase bearing life in this difficult application. These include marine bearings (metal shaft with a rubber stationary member), ceramic sleeve bearings, tungsten carbide sleeve bearings and metallic sleeve bearings. For the most part, these various techniques have been unsuccessful due to reasons previously mentioned. The materials that are wear resistant (i.e. ceramics and tungsten carbides) are too brittle to handle the impact requirements. Alternatively, the tougher materials such as steel and rubber do not have sufficient wear resistance. The inherent brittleness of carbides and ceramics makes them subject to chipping. Dislodging and fracturing of the pieces can damage the remainder of the bearing and other components within the motor.
Direct applied wear protection techniques such thermal sprays and brazing of individual carbide inserts have also been used with limited success. These techniques are prone to spalling and delamination due to inferior bond strengths. Spray techniques are also difficult to apply on the internal surfaces of the stationary members.
One process which has been relatively successful in the wear protection of the radial bearing is the Conforma Clad process. This process, as covered in U.S. Pat. No. 3,743,556, which is hereby incorporated by reference, utilizes powder cloth technology to produce a coating that is tough, wear resistant, and metallurgically bonded to the bearing substrate. The main drawback to the Conforma Clad process is that it is expensive, and the coating is applied at high temperatures which leads to distortion due to differences in coefficients of expansion between the coating and substrate.
Another limitation is cracking of the coating during heat treatment to increase the toughness of the supporting base steel. For example, bearings for oil-drilling mud motors and rolls for forming metals require the combination of resistance to wear and metallurgical toughness to withstand impact stresses. Typically, wear-resistant metals and metaloids have a Rockwell C hardness of over 60, whereas tough steels have Rockwell C hardness of below 50 and often below 40. Heat-treating steel to obtain the high hardness needed for wear resistance makes it too brittle to resist high impact loads.
This problem has been approached by applying a wear-resistant coating over heat-treatable steel, such as the more common heat-treatable steels like the AISI 4100 series steels that develop toughness through the formation of martensite. Martensite expands during heat treatment. Thus, heat treating a martensitic steel that has been coated with a wear-resistant coating generates cracks in the coating. This has been a disadvantage of the carbide composite coatings applied to mud-motor bearings. Even with cracks, coatings still provide adequate protection, but performance would be improved without the cracks.
Replaceable inside-diameter sleeves with composite carbide coatings are used in extruder barrels. However, they are not subject to high impact stresses as some bearings, shafts, and rolls are. So the need to configure the mechanical properties of the sleeve to those of the steel base material to maximize impact resistance is not as great.